King Harald Fairhair's use of berserker "shock troops" became a sphere of influence. Other Scandinavian kings used berserkers as part of their army of hirðmen and sometimes ranked them as equivalent to a royal bodyguard. It may be that at least some of those warriors just adopted the organization or rituals of berserk warbands or used the name as a deterrent or claim of their ferocity.

The earliest surviving reference to the term berserker is in Haraldskvæði, a skaldic poem composed by Thórbiörn Hornklofi in the late ninth century in honour of King Harald Fairhair, the famous ruler of Norway. The poem was preserved by Snorri Sturluson. In this poem, Harald's army includes a warrior gang of berserkers fighting under his name at the battle of Hafrsfjord. In it, they are described as Ulfheðnar = "men clad in wolf skins". This grounds a connection between bears and wolves in Norse warrior culture and the common assumption that the word "berserker" itself originates from men wearing the skin of the bear. An alternative etymology is from "bare", meaning unencumbered by a mail shirt.

Snorri Sturluson goes on to mention berserkers in the Ynglinga Saga: "his Odin's men rushed forward without armor, were as mad as dogs or wolves, bit their shields, and were as strong as bears or wild bulls, and killed people at a blow, but neither fire nor iron told upon themselves" (Ch. 6). Berserkers appear prominently in a multitude of other sagas and poems including The Saga of Hrólf Kraki, many of which describe berserkers as ravenous barbarians who loot, plunder, and kill indiscriminately.

Much can be derived about berserkers from Egils saga. Egil's grandfather was named Kveld-Ulf meaning "evening wolf", and this is generally ascribed as meaning he was a werewolf. Kveld-Ulf's son, referred to as Skalla-Grimm, was a berserker. Kveld-Ulf and Skalla-Grimm are both depicted as irascible and violent throughout the saga, the latter attempting to kill his son. Egill Skallagrímsson himself is described in the saga as attacking opponents with his teeth (namely when he ripped out a berserker's jugular vein during a duel). Patently, violence and gruesome tragedies permeate the berserker ethos described in Icelandic sagas such as this one.

Theories about what causes berserker behavior include ingestion of materials with psychoactive properties, psychological processes, and medical conditions.

According to a theory of spirit possession, the berserk rage was achieved through possession by the animal spirit of either a bear or a wolf. Berserkers would cultivate an ability to allow the animal spirit to take over their body during a fight. This is seen as a somewhat peculiar application of animal totemism.

Botanists have suggested the behavior might be tied to ingestion of bog myrtle (Myrica gale syn: Gale palustris), a plant that was one of the main spices in alcoholic beverages in Scandinavia. The drawback is that it increases the hangover headache afterwards. Drinking alcoholic beverages spiced with bog myrtle the night before going to battle might have resulted in unusually aggressive behavior.

The notion that Nordic Vikings used the fly agaricmushroom to produce their berserker rages was first suggested by the Swedish professor Samuel Ödman in 1784. Ödman based his theory on reports about the use of fly-agaric among Siberian shamans. The notion has become widespread since the 19th century, but no contemporary sources mention this use or anything similar in their description of berserkers. In addition, the injection of bufotenine from Bufo marinus toad skin into humans was shown to produce similar symptoms to the "Berserker" descriptions. These findings, first examined by Howard Fabing in 1956, were later linked to the induction of zombie characteristics by ethnobotanists in 1983.

A simple theory attributes the behavior to drunken rage. It is also possible that berserkers worked themselves into their frenzy through purely psychological processes, perhaps using frenzied rituals and dances. According to Saxo Grammaticus they also drank bear or wolf blood.

U.S. professor Jesse L. Byock claims (in Scientific American, 1995) that berserker rage could have been a symptom of Paget's disease. Uncontrolled skull bone growth could have caused painful pressure in the head. He mentions the unattractive and large head of Egill Skallagrímsson in Egilssaga. Other possibilities are mild epilepsy, rabies, and hysteria.

Manic depressive (bipolar disorder) offers another possible explanation for this behavior. During a manic state, the person affected is impulsive and often seems uncontrolled, and often perceived as exhibiting aggressive behaviour. It would however be impossible to control these states and simultaneously induce a manic state in a large group of people as and when required.

Nevertheless, these theories are highly unlikely, as the berserkers would—seemingly—inevitably turn against each other instead of the enemies. During battle, they are consistently described in the frenzy of rage; yet, the berserkers, while sometimes purportedly felling allies, seem to have avoided attacking each other.

Hilda Ellis-Davidson draws a parallel between berserkers and the mention by the Byzantine emperor Constantine VII in his book De cerimoniis aulae byzantinae ("Book of Ceremonies of the Byzantine court") of a "Gothic Dance" performed by members of his Varangian Guard (Norse warriors working in the service of the Byzantine Empire), who took part wearing animal skins and masks: she believes this may have been connected with berserker rites. (Hilda R. Ellis-Davidson. Pagan Scandinavia. NY: Frederick A. Praeger. 1967. p. 100)

Similar behaviour is described in the Iliad, where warriors who are "possessed" by a God or Goddess exhibit superhuman powers.

Some aspects of the Malay phenomenon of running amok bear a close resemblance to berserkergang.

Among the Irish, Cúchulainn acted in the 'battle frenzy', or 'contortion', and many other famous Irish warriors from the pre-Christian period became possessed and frenzied. They are described in texts such as The Tain as foaming at the mouth and not calming down after battle until doused with cold water.